firefly wrote:
Explain the physics on how 2 planes can take down 3 towers? The small bit of fire on B7 was hardly enough to cause that spectacular controled demolition type fall.

Actually, it was enough, at least according the 3-year study done by National Institute of Standards and Technology who probably have a bit more knowledge and expertise on these matters than either of us.

Sun Jan 09, 2011 2:10 pm

Dan Shay

Joined: 30 Aug 2003
Posts: 11246
Location: MN

The only plausible conspiracy theory regarding 9/11 is that flight 93 over Pennsylvania was shot down because there were 3 nuclear power plants and a White House in the general vicinity.

Sun Jan 09, 2011 2:19 pm

firefly

Joined: 27 Sep 2002
Posts: 3990
Location: Montreal

And once again, nobody can explain it.

Who needs to think for yourself when there's an institution that does all the thinking for you.

Sun Jan 09, 2011 2:30 pm

mancabbage

Joined: 29 Jun 2005
Posts: 9267
Location: london

huh? them shits fell down because two big fucking planes went into them. I saw it on the tv mate. Never played jenga?

Sun Jan 09, 2011 2:57 pm

xGasPricesx

Joined: 23 May 2008
Posts: 1591

firefly wrote: And once again, nobody can explain it.

Who needs to think for yourself when there's an institution that does all the thinking for you.

Round and round, in circles we go.

Sun Jan 09, 2011 3:06 pm

xGasPricesx

Joined: 23 May 2008
Posts: 1591

mancabbage wrote: huh? them shits fell down because two big fucking planes went into them. I saw it on the tv mate. Never played jenga?

He's (hopefully) only talking about Building 7 here though, which wasn't hit by a plane but had also collapsed a few hours after the Twin Towers did.

Sun Jan 09, 2011 3:08 pm

T-Wrexp00ny tang

Joined: 30 Jun 2002
Posts: 6409
Location: Detroit, Michigan

The only 9/11 conspiracy that I believe is that Bush and Cheney willfully ignored Condaliza's warnings because they wanted it to happen...
...they just didn't expect things to go so bad.

The PNAC was looking for an excuse to invade Iraq...

The system was blinking red and they were fashioned with rose shade glasses.

Sun Jan 09, 2011 3:12 pm

iammessage

Joined: 10 Aug 2009
Posts: 655
Location: ZOO DIRT, OH

rappers, illuminati, 9-11, boom.

Sun Jan 09, 2011 4:17 pm

breakreephomophobic yet curious

Joined: 27 Sep 2004
Posts: 6627
Location: Fifth Jerusalem

Here you go, firefly. This is an article written less than a year after the collapse. It discusses the fires exclusively and is written in a very clear and simple manner. Sadly I had to copy-paste from a PDF, but I'll make it easier by bolding the parts I think are most relevant to your question.

Quote:
Fire resistance of framed
buildings*
Ian Burgess
University of Sheffield, UK
Abstract
Until recently few buildings that experienced serious fires collapsed as a
result of those fires. However, the dramatic and tragic collapse of the twin
towers of the World Trade Center last September, which was due not to the
initial impacts but to the fires created by the aviation fuel, has prompted
close examination of the way in which buildings can fail in fires. The effects
of fires on multi-storey buildings are examined here and may allow us to
construct safer buildings in the future.
(Some figures in this article are in colour only in the electronic version)
The attack on the twin towers of the World
Trade Center in New York on 11 September 2001
brought into the public eye the hazards that fire
can pose to major building structures. The twin
towers themselves, known as WTC1 and WTC2,
withstood the massive local damage caused by
impact from Boeing 767-200 aircraft, but fell some
time later after fires ignited by the jet fuel had
burned unchecked around the impact zones. The
collapse of the towers, and the loss of life that
was caused, has tended to eclipse the fact that
the 47-storey WTC7 also collapsed several hours
later. This building had experienced some damage
caused by the debris from the twin towers, but
the key aspect was that so many firemen had been
killed in the collapse of Buildings 1 and 2 that the
New York Fire Brigade was unable to fight fires
started by the initial damage.
Although lives are regularly lost in fires in
buildings they are very rarely lost because of
structural collapse. Most deaths occur in the
home, and the main hazard to occupants of any
kind of building in fire is ingesting smoke. These
deaths occur quite early in a typical fire, when the
* The contents of this article were presented at the Institute of
Physics Annual Congress held in Brighton in April 2002.
temperatures of the load-bearing structure are still
too low for collapse to be an issue.

Fire temperatures

The way a fire develops is controlled by a
three-way balance of fuel, oxygen supply and
temperature. When water hoses are used to fight a
fire the main effect is to cool the atmosphere and
the structure, which inhibits the reaction between
fuel and oxygen. Alternatively, CO2 extinguishers
replace oxygen in the vicinity of the fire for
long enough to kill the reaction. In a typical
building fire the relationship between atmospheric
temperature and time goes through a sequence of
phases (figure 1). During the ignition phase there
is very little immediate danger. This is followed by
a period of relatively slowgrowth, in which the fire
stays very localized although significant amounts
of smoke may be generated. The transition from a
local to a fully developed fire is generally sudden,
at a ‘flash-over’ when the flame reaches the ceiling
of the compartment and all the organic material
present starts to burn. The fire temperature then
rises rapidly until it reaches a peak when heat
input is balanced by heat losses, and then falls
as fuel is consumed. After flash-over effective
fire-fighting is almost impossible, and fire brigades
usually concentrate on preventing spread to other
buildings or to other storeys of the same building.
Afire’s temperature growth is very sensitive to
the amount, nature and distribution of combustible
material, the ventilation characteristics of the fire
compartment, and the materials of the surrounding
surfaces. It is very difficult to predict, and for
convenience a standard time–temperature curve is
adopted internationally against which building fire
resistance ratings are measured. This standard
fire curve starts at flash-over and its temperature
increases indefinitely, making it completely
unrealistic as a model of a real fire.

Multi-storey buildings in fire

Buildings can be constructed in many different
ways. The system adopted for a particular
building in a particular place depends on its use
and a combination of local economic factors and
statutory requirements, as well as the willingness
of architects, engineers, occupants and control
authorities to accept unfamiliar systems. Modern
multi-storey construction tends to use a structural
frame, effectively the building’s skeleton, to carry
the floors. This allows large numbers of storeys in
a building and leaves almost complete flexibility
to its owners to partition each storey as they
wish. Structural frames may be constructed
from reinforced concrete or steel. In Britain and
North America the great majority of such frames
use structural steelwork systems, because these
are extremely efficient to build. In composite
construction, the most common of the current
systems, the concrete floor slabs are continuously
connected to the beams of the structural frame,
creating a very robust structure in which local
loads are shared among a range of members as
well as those that directly support them.

All common building materials lose strength
when heated to high temperatures. For steel the
change can be seen in the stress–strain curves
(figure 2) at temperatures as low as 300 ◦C.
Although steel does not melt until about 1500 ◦C,
only 23% of the ambient-temperature strength
remains at 700 ◦C. At 800 ◦C this has reduced
to 11% and at 900 ◦C to 6%. Concrete also loses
strength as its temperature increases, because of
a combination of internal cracking and chemical
changes. As a brittle material the stress–strain
curves for concrete (figure 3) are different in
form from those for steel. The curves all have
a maximum compressive strength, followed by
a rapidly descending branch, and little tensile
strength.

Fire resistance requirements

Fire safety requirements for buildings are set
by law, and attempt to reflect the risks that
fires pose to the occupants and to fire-fighters.
The general fire safety scenario for a building
includes provision of adequate means of escape
for occupants, fire detection and control, together
with a fire resistance requirement for the structure
which is expressed as a period of time. For
example, a hospital, which contains patients who
need to be moved with great care, would have
a four-hour fire resistance rating, while a lowrise
office building might only require 30 minutes.
It is not generally appreciated that this is not a
guaranteed survival time for the structure if a fire
occurs. It actually relates to the behaviour of the
structure as if it were subjected to the international
standard heating curve.
The most effective form of fire resistance is
fire prevention. This is possible using suitable
temperature and smoke detection systems, which
can be linked directly to the local fire brigade.
These ‘active’ systems include sprinklers, which
are activated by atmospheric temperatures well
below the flash-over point. They are surprisingly
effective in suppressing local fires before they
have a chance to become fully developed, and if
maintained properly have a reliability of over 99%.
The most usual way of achieving the fire
resistance requirement for a steel-framed structure
is simply to cover the exposed steel with a
prescribed thickness of an insulating material.
The most common insulation materials are fireresistant
boarding, cementitious sprays, and
intumescent paints which expand to form a thick
lightweight coating when heated to about 300 ◦C.
The required thickness of the alternative systems is
specified by the protection manufacturers, and has
the sole objective of keeping the steel temperature
below a fixed critical temperature (usually 550 ◦C)
within the fire resistance period, irrespective of
the loading level. The fixed critical temperature
is based on reduction of steel strength until the
normal design safety factors on fully loaded
members have been fully eroded. A more rational
performance-based approach has been introduced
recently which relates the critical temperature of
any member to its load level. This is logical
because a lightly loaded member does not fail
until it reaches a very high temperature, and will
therefore survive much longer than a fully loaded
member. In a normal building, in which members
have a range of load levels that are all well below
the maximum loadings for which the members are
designed, the performance-based approach leads
to lower requirements for fire protection than the
prescriptive approach, and is clearly more realistic
in at least this sense.
The high degree of continuity in composite
framed buildings means that local loads are shared
by parts of the structure that are quite remote
from the loads themselves. In fire, when the
loss of strength and stiffness of heated parts is
happening at the same time as restrained thermal
expansion, this usually causes the inherent fire
resistance of a real building to be much higher
than is indicated by these methods based on
members with no interaction. The only way of
predicting how a continuous building behaves in
fire is by advanced numerical modelling which
includes all the changes in material behaviour at
high temperature, the distributions of temperature
through parts of the structure and the ability
to work accurately at very high deflections and
strains. When designers have the expertise to use
such methods they give the most realistic view of
the way the structure will behave during a fire,
and generally show that much less protection is
necessary than the cruder methods suggest.
Different types of local failure cause different
degrees of risk to a building. Failure of a column
by buckling is potentially disastrous, because the
whole region of the building above it will move
downwards (figure 4) with the top of the failing
column, and even if the weight of this part of
the building could be redistributed to adjacent
columns then they may have to be even stronger
to kill the huge amount of kinetic energy that
may accumulate. The surrounding columns can
be overloaded by the static and dynamic forces
caused by decelerating the motion of this huge
mass of building, and a progressive failure can
develop rapidly. It is not sensible to leave columns
unprotected because the potential consequence is
so extreme. Beams can weaken and undergo
very large deflections (figure 5), but provided
that they remain connected at their ends, and that
the attached slabs do not collapse, all the effects
will usually remain local and there is no building
failure. If floor slabs fail locally as a result of high
deflection, the main problem is that fire can then
spread upwards into the next storey; containment
of a fire so that it can burn out locally is one of the
main objectives of fire safety engineering. If joints
between beams and columns fracture (figure 5),
then several undesirable effects can occur, the
least of which is that the fire can again spread
upwards through the gap. In addition, if beams
lose support then the floors supported by them
collapse onto the floor below, which may not be
strong enough to carry the increased load and could
start a progressive collapse of floor slabs down
the building. Another effect may be to double the
unsupported height of the column to which the
joint is connected, and this considerably reduces
the strength of the column, which may then fail.

Fires in multi-storey buildings

The sheer cost of full-scale fire testing on real
buildings means that until recently only individual
members have been tested in fire, and even these
tests have been on unrealistically short members.
Figure 6. One Meridian Plaza, Philadelphia.
Accidental fires can give clues to the behaviour
patterns, but it is usually impossible to reconstruct
the full scenario including the temperature
development and fire spread. However, several
well-documented fires in recent years have given
a generally optimistic picture of the inherent
fire resistance of steel framed buildings. Two
significant examples are:
• One Meridian Plaza in Philadelphia (figure
6) experienced a major fire on 23–24
February 1991. The fire started on the 22nd
floor of the 38-storey building and spread upwards,
mainly by flames from broken windows
reaching the windows on the storey
above. There was no sprinkler system on
the 21st to the 29th storeys. The local fire
brigade fought the fire for 11 hours, but conditions
in the building were very hazardous and
three fire-fighters died. The brigadewas withdrawn
from the building and the fire burned
unchecked for a further eight hours, consuming
the contents of nine floors. On the 30th
floor a sprinkler system had been installed,
and further fire spread was stopped. There
was no structural collapse during the fire, and
the building stood for several years afterwards
in its damaged state. The key aspect in this
case is that none of the protected columns
failed, although beams were very highly distorted.
• Broadgate Phase 8 in London was fully
constructed, but had not been fire-protected
September 2002 PHYSICS EDUCATION 393
I Burgess
Figure 7. Broadgate Phase 8: London, 1990.
(Photograph courtesy London Fire Brigade.)
when a major fire broke out in contractors’
equipment in 1990. The fire burned in
an open-plan storey of the building for
2 hours, causing total damage estimated
at £20 million. The unprotected internal
columns sustained quite extensive damage,
shortening by as much as 200 mm, and the
long-span beams supporting the upper floor
were very highly distorted (figure 7). The
external columns, which were not as severely
affected, stayed intact and stabilized the
building. Surprisingly, the cost of structural
repairs after the fire (propping and cutting
out the distorted steelwork, jacking-up the
upper floors which hadmoved downwards and
replacing the removed members) was only
£11
2 million.
Because of a widespread feeling after similar
experiences of accidental fires that steel framed
structures are actually much more fire-resistant
than the existing design methods allow, a large
experimental project was run in Britain in the mid-
1990s. A full-scale eight-storey building, typical
of modern medium-rise office structures (although
there was no need to finish it architecturally or
to provide building services), was constructed
inside a massive hangar at Cardington which
had originally been built to house airships in the
1920s. All the floors were loaded with sandbags
to a level typical for offices in the UK. Six
fire tests were performed on the building, with
several hundred measurements of temperatures,
deflections and strains being taken during and after
the fires. The largest test covered a floor area of
380m2, the largest fully instrumented fire test ever
performed. The columns were covered with box
protection because of the severe consequences of
a column failure, but all the internal steel beams
were left completely unprotected. Some of the
fires were lengthy and severe; the temperatures of
steel beams usually reached over 800 ◦C, and in
one test 1150 ◦C, at which the steel retains about
2% of its design strength. However, despite the
fact that these temperatures were well above the
critical temperatures given by the normal design
procedures, and the distortions of floors were quite
high, no failures occurred in any of the tests.
Since the tests finished in 1996 the data produced
have stimulated an upsurge in research aimed at
understanding the complex interactions that take
place during a building fire and producing reliable
numerical models to predict the performance of
future projects at the design stage.
TheWorld Trade Center—11 September
2001
The terrorist attacks on the twin towers of the
World Trade Center in New York, followed by
their collapse and the deaths of 2800 people, have
already had tremendous repercussions in political
and international terms, and the aftershocks
will undoubtedly continue for some time. On
the technical front, one important question has
been why the buildings collapsed after they had
apparently withstood impacts by Boeing 767-
200 aircraft but after the impact floors had been
engulfed in fire. These disastrous building
collapses were unprecedented, and completely at
odds with the experiences of major building fires
in recent years.
The twin towers, known as WTC1 and
WTC2, were designed in the late 1960s, using an
innovative structural system, still almost unique
in tall building construction. This was influenced
by a desire to use production-line techniques of
construction, using pre-fabricated modules that
were as large as possible, assembled in kit form
on site. Wind loads were to be resisted without
the usual systems of diagonal bracing members
or a strong reinforced concrete box enclosing
the service core, but by a ‘pierced tube’ of
closely spaced external columns rigidly connected
together at each storey level by very deep beams.
The floor system consisted of parallel long-span
steel trusses connected together by corrugated
steel decking onto which the concrete slabs were
cast on site. The slabs were positively connected
to the trusses by allowing the diagonal bracing to
be cast into the concrete. Fire protection to the
floor trusses and most of the external steelwork
was provided by a sprayed coating of lightweight
cementitious material containing mineral fibres.
The service core of the building consisted of
vertical steel columns designed simply to carry
the vertical forces from the inner ends of the
floor trusses. The service cores contained the
emergency stairs as well as the extensive system
of lifts. Since these stairs are the escape route for
occupants they have to be fire-protected, and this
was done by attaching ‘dry-wall’ (insulating board
similar to plasterboard) around the service cores.
Neither of the fire protection materials used is
particularly resistant to impact or to flying debris.
The layout of the structure is shown in figure 8 and
the main structural modules used in construction in
figure 9. The towers were nearly identical, at 110
storeys above ground, but their rectangular service
core areas were aligned at 90◦ to each other.
The WTC complex actually contained seven
buildings (figure 10), of which only WTC7 (a
47-storey office building constructed in the mid-
1980s) was just outside the city block occupied by
all the other buildings. Of the others WTC3 was a
22-storey hotel, and WTC4 to WTC6 were eightor
nine-storey office buildings.
The main events of 11 September were (all
times EDT):
08.46 WTC1 struck by Boeing 767-200 at
approximately 470 mph, centrally on
North face between floors 94 and 98
09.03 WTC2 struck by Boeing 767-200 at
approximately 590 mph, off-centre on
South face between floors 78 and 84
09.59 WTC2 collapses
10.28 WTC1 collapses
17.20 WTC7 collapses.
In the cases of WTC1 and WTC2 the impact
damage was understandably localized (the mass
of an aircraft would have been somewhat under
200 tonnes while the mass of each tower would
have been about 200 000 tonnes) but this local
damage to structural memberswas obviously quite
extensive. Between 27 and 36 exterior columns
are shown by the photographic evidence to have
had portions removed over a height of 4–5 storeys
in each of the buildings. The off-centre impact
on WTC2, which was also aligned so that the
short width of its central core was exposed to
the impact direction, might have resulted in less
damage to the core columns, though the much
higher impact velocity would make damage to
impacted columns more severe. This, however, is
all based on speculation, because the photographic
evidence is insufficient to provide any firm clues to
the interior structural damage of either tower. It is
clear that WTC2 suffered an asymmetric pattern
of column failure, since video footage (figure 11)
shows the upper storeys having rotated by about
15◦ from the vertical before the whole building
fell vertically. All indications fromWTC1, whose
100mtall TV transmission tower is clearly visible
in the video evidence of the collapse, are that
the building fell very nearly vertically, without
any significant rotation of the upper part towards
the damaged zone, indicating that the initiating
event was failure of the columns in the core of the
building.
The fireballs caused by the WTC2 impact
(figure 12) grew to their maximum size in about
two seconds. This is much too slow an expansion
of the fuel vapour mixture to be considered as
an explosion on impact, so it seems certain that
structural damage was not caused by an explosive
shock-wave inside the building. Research on
similar fireballs suggests that as much as 12 tonnes
of fuel may have been consumed in the fireball, out
of the assumed 40 tonnes in each aircraft’s tanks.
If half of the remainder was distributed around the
impact floors, then the energy contained in this
fuel is a very minor part of the total fuel load
on these floors, and would have been consumed
within a few minutes. However, since it had
almost simultaneously generated a post-flashover
fire over the whole of the four or five impact floors,
the office contents were then capable of burning
for considerably longer, and so the structures were
given the time they needed to reach very high
temperatures.
There is no doubt that the events which
initiated the fall of the storeys above the impact
zones in both cases were column failures, probably
happening individually and throwing extra load
onto the adjacent columns, which were at very
similar temperatures and therefore unable to carry
this increased loading. A ‘ripple-effect’ would
then be created very rapidly, with all columns at
the level failing as the upper storeys began tomove
downward. As this progressed, floor truss connections
would fail due to the weight of debris from
above and an internal progressive collapse of the
floor slabs would happen ahead of the progressive
column failure. The question that remains open is
which of two possible scenarios caused the initial
column failures. Both assume that nearly all the
lightweight fire protection material covering the
steelwork had been removed by the impact and
flying debris:
Scenario 1. Columns with their loading considerably
increased by the removal of several
adjacent columns by the impact are critically
weakened by a combination of impact damage
which may have bent them, and a doubling
or trebling of their original length due to
the failure of floor-truss connections as part of
the original impact damage. When their steel
temperature has risen sufficiently their load
capacity is reduced below their actual loading.
Scenario 2. Floor trusses heat very rapidly since
they are constructed of thin members and
have lost their insulation. The trusses
collapse, hanging like suspension cables
between their connections at the core and
perimeter columns. This causes high pulling
forces on the connections, which were not
designed to withstand such forces. One
such connection with a core column fails
and the beam hangs down, overloading the
next connection, and a progressive failure of
the line of connections takes place, dropping
the floor slab and any debris it is supporting
onto the slab below. This leaves heated
columns with an increased effective length,
as in Scenario 1, and they fail.
Visually each of the twin towers appeared
to fall within its own footprint, but in fact
the zone of heavy debris extended up to 50
metres from the buildings, and all the minor
WTC buildings were damaged by falling material.
WTC3, the 22-storey Marriott Hotel which was
within the debris zone of both towers, was of
much more conventional framed construction than
the towers. It was virtually crushed in areas
where huge amounts of material fell onto it but
arrested the collapse and protected occupants
on its lower floors. The three relatively lowlevel
office buildings, WTC4 to WTC6, were
struck by considerable amounts of debris. Most
of WTC4 collapsed under impact from the
exterior columns of WTC2, whereas WTC5 and
WTC6 experienced large local collapses and very
extensive fires. However, the fires in these
buildings, which were not fought, did not cause
overall collapse of the buildings.
WTC7, a 47-storey office building north of
the main complex, was the least affected of the
buildings by impact damage from the fall of
the twin towers. There is no evidence of any
damage to the roof or the three sides facing
away from the WTC complex. However, when
WTC1 collapsed there was impact damage to
several floors on the south face, and several fires
were observed to be present on different floors.
The fires were not fought, largely because of the
very large number of casualties that had been
sustained by the New York Fire Department in
the collapses of the twin towers. The building
was sprinklered, but the only main water supply
had been fractured by the debris, and there
would not have been enough water on site to
combat the fire growth. The collapse of this
building seven hours after that of WTC1 seems
to have been due to complex causes and still
needs considerable forensic engineering work.
However, it seems significant that the building
was constructed over a major existing electricity
supply sub-station building using massive longspan
trusses known as ‘transfer structures’ to span
over and around the existing buildings between
the 5th and 7th floor levels. These supported
the columns from this level to the roof. Another
significant fact is that the building contained large
stores of diesel fuel used to power emergency
generators in various parts of the building. The
pumps supplying these generators would have
operated automatically when the main power was
interrupted, and could have continued to supply
fuel oil to fires if the supply pipes were fractured;
investigators have found a net loss of about 50
tonnes of fuel oil from the supply tanks. Clues
to the events which initiated failure are given by
the two photographs of the building (figure 13)
taken in sequence before and during the collapse.
A penthouse structure on the roof of the building
is clearly visible in the first photograph but has
disappeared very shortly afterwards although the
building shell appears intact. The penthouse was
supported by columns originating at two transfer
trusses on Level 5, and this ‘implosion’ of the
building is typical of demolition techniques used to
remove tall buildings in crowded inner-city areas,
in which the objective is to keep debris within the
building footprint. It seems likely that the transfer
structures failed in fire, allowing the supported
columns to fall, pulling the rest of the building
structure inwards.
Conclusions
In a sense the fall of the twin towers and WTC7
after severe fires has reversed a general theme
in structural fire engineering which has tended
to show that modern composite framed buildings
are much more fire-resistant than has traditionally
been assumed in design. However, the progress in
the past two decades has really been in beginning
to understand how buildings behave in fire, and
starting to use this in place of the very superficial
design philosophies that had previously been used.
Major advances in structural engineering have
often had their origins in major disasters, and some
concentration on understanding and designing
against the effects that caused the WTC tragedy
may, perversely, lead to buildings that are both
more effective and more efficient in resisting fire.
Received 12 July 2002
PII: S0031-9120(02)39422-X
Ian Burgess is Professor of Structural
Engineering at Sheffield University. In
the mid-1980s he began a collaboration
that attempts to develop numerical
techniques for modelling of the
behaviour of steel and composite
elements in fire. This research
programme now focuses on developing
more rational design principles for
structural fire engineering.
September 2002 PHYSICS EDUCATION 399

Sun Jan 09, 2011 9:04 pm

xGasPricesx

Joined: 23 May 2008
Posts: 1591

breakreep wrote: Here you go, firefly. This is an article written less than a year after the collapse. It discusses the fires exclusively and is written in a very clear and simple manner. Sadly I had to copy-paste from a PDF, but I'll make it easier by bolding the parts I think are most relevant to your question.

Quote:
*Insert long-form propaganda here

Nice try, buddy. But everyone knows that scientific data and facts are just another form of government control. If you want to continue to let the institutions think for you, then fine, but I have the courage to think for myself and to rely on a little something called "the gut".

Sun Jan 09, 2011 9:14 pm

breakreephomophobic yet curious

Joined: 27 Sep 2004
Posts: 6627
Location: Fifth Jerusalem

While we're on the subject, people who like making up problems where none exist often complain that the two primary towers fell too quickly to be explained by the impacts and gravity.

Here are the abstract and conclusion from a much more recent (2008) article which is also much more technical. I hope you will see from the language and mathematics involved why physicists and engineers don't spend much time trying to distill this stuff to people who don't want to believe them in the first place: Not only does the material take work to understand, but it is written in a language that you cannot understand without advanced math courses, and most physicists and engineers aren't trained how to translate that stuff into the simple sound-byte English that antagonistic audiences might listen to. Some of the symbols used in the abstract and conclusion won't even paste here, they just turn up as squares. The body of the paper is filled with differential equations; if you don't know what those are, then you can't read them anyway even if I posted them and they didn't look like rows of squares. Enjoy.

Quote: What Did and Did Not Cause Collapse of World Trade Center
Twin Towers in New York?
Zdeněk P. Bažant, Hon.M.ASCE1; Jia-Liang Le2; Frank R. Greening3; and David B. Benson4
Abstract: Previous analysis of progressive collapse showed that gravity alone suffices to explain the overall collapse of the World Trade
Center Towers. However, it remains to be determined whether the recent allegations of controlled demolition have any scientific merit.
The present analysis proves that they do not. The video record available for the first few seconds of collapse is shown to agree with the
motion history calculated from the differential equation of progressive collapse but, despite uncertain values of some parameters, it is
totally out of range of the free fall hypothesis, on which these allegations rest. It is shown that the observed size range 0.01–0.1 mm of
the dust particles of pulverized concrete is consistent with the theory of comminution caused by impact, and that less than 10% of the total
gravitational energy, converted to kinetic energy, sufficed to produce this dust whereas, more than 150 t of TNT per tower would have
to be installed, into many small holes drilled into concrete, to produce the same pulverization. The air ejected from the building by
gravitational collapse must have attained, near the ground, the speed of almost 500 miles per hour or 223 m/ s, or 803 km/h on average,
and fluctuations must have reached the speed of sound. This explains the loud booms and wide spreading of pulverized concrete and other
fragments, and shows that the lower margin of the dust cloud could not have coincided with the crushing front. The resisting upward
forces due to pulverization and to ejection of air, dust, and solid fragments, neglected in previous studies, are indeed found to be negligible
during the first few seconds of collapse but not insignificant near the end of crush-down. The calculated crush-down duration is found to
match a logical interpretation of seismic record, while the free fall duration grossly disagrees with this record.

...

Conclusions
Several of the parameters of the present mathematical model have
a large range of uncertainty. However, the solution exhibits small
sensitivity to some of them, and the values of others can be fixed
on the basis of observations or physical analysis. One and the
same mathematical model, with one and the same set of parameters,
is shown to be capable of matching all of the observations,
including: 1 the video records of the first few seconds of motion
of both towers; 2 the seismic records for both towers; 3 the
mass and size distributions of the comminuted particles of concrete;
4 the energy requirement for the comminution that occurred;
5 the wide spread of the fine dust around the tower; 6
the loud booms heard during collapse; 7 the fast expansion of
dust clouds during collapse; and 8 the dust content of the cloud
implied by its size. At the same time, the alternative allegations of
some kinds of controlled demolition are shown to be totally out of
range of the present mathematical model, even if the full range of
parameter uncertainties is considered.
These conclusions show the allegations of controlled demolition
to be absurd and leave no doubt that the towers failed due to
gravity-driven progressive collapse triggered by the effects of fire.

Also from the exact same article, here are some choice pieces of text near the beginning which discuss the fires specifically, before moving on to the main discussion of gravitational collapse:

Quote: Note, though, that only
1% of the columns from the fire stories were examined. Consequently,
NIST cautioned that the findings from paint cracking test
and annealing studies are not indicative of the steel temperatures
in the fire stories. Thus, although very high steel temperature are
likely, there is no direct evidence. But are high steel temperatures
really necessary to explain collapse?
Not really. The initial speculation that very high temperatures
were necessary to explain collapse must now be revised since
tests revealed a strong temperature effect on the yield strength of
the steel used. The tests by NIST 2005, part NCSTAR 1-3D,
p. 135, Fig. 6-6 showed that, at temperatures of 150, 250, and
350°C, the yield strength of the steel used in the fire stories
decreased by 12, 19, and 25%, respectively. These reductions
apply to normal durations of laboratory strength tests up to several
minutes. Since the thermally activated decrease of yield
stress is a time-dependent process, the yield strength decrease
must have been even greater for the heating durations in the towers,
which were on the order of 1 h. These effects of heating are
further documented by the recent fire tests reported by Zeng et al.
2003, which showed that structural steel columns under a sustained
load of 50%–70% of their cold strength collapse when
heated to 250°C.
Although a detailed computer analysis of column stresses after
aircraft impact is certainly possible, it would be quite tedious and
demanding, and has not been carried out by NIST. Nevertheless,
it can be easily explained that the stress in some surviving columns
most likely exceeded 88% of their cold strength 0. In that
case, any steel temperature 150°C sufficed to trigger the viscoplastic
buckling of columns Bažant and Le 2008. This conclusion
is further supported by simple calculations showing that if,
for instance, the column load is raised at temperature of 250°C
from 0.3Pt to 0.9Pt where Pt=failure load=tangent modulus
load, the critical time of creep buckling Bažant and Cedolin
2003, Chap. 8 and 9 gets shortened from 2,400 to 1 h note that,
in structural mechanics, the term “creep buckling” or “viscoplastic
buckling” represents any time-dependent buckling; on the
other hand, in materials science, the term “creep” is reserved for
the time-dependent deformation at stresses 0.50, while the
time-dependent deformation at stresses near 0 is called the
“flow” Frost and Ashby 1982.
Therefore, to decide whether the gravity-driven progressive
collapse is the correct explanation, the temperature level alone is
irrelevant Bažant and Le 2008. It is meaningless and a waste of
time to argue about it without calculating the stresses in columns.
For low stress, high temperature is necessary to cause collapse,
but for high enough stress, even a modestly elevated temperature
will cause it.
The fact that, after aircraft impact, the loads of some columns
must have been close to their strength limit can be clarified
by Fig. 1. The asymmetry of aircraft impact damage caused
the stiffness centroid of the story to acquire a significant eccentricity,
e Fig. 1b. The corresponding bending moment Pe
of gravity load P=m0g m0=mass of the initial upper falling
part; and g=gravity acceleration caused nonuniform axial
shortening and additional axial stresses in the surviving columns
Figs. 1e and h, which raised the stresses in the columns on the
weaker side of the story much above the average stress due to
gravity. The subsequent heating weakened the overloaded columns
on the weaker side left side in the figure more than those
on the stronger side, and gradually caused more and more of them
to lose their load-carrying capability. This further enlarged e and
thus increased the nonuniformity of column deformations and
stresses Fig. 1f and i, until the buckling of a sufficient number
of columns led to the overall stability loss.

Sun Jan 09, 2011 9:28 pm

breakreephomophobic yet curious

Joined: 27 Sep 2004
Posts: 6627
Location: Fifth Jerusalem

xGasPricesx wrote: Nice try, buddy. But everyone knows that scientific data and facts are just another form of government control. If you want to continue to let the institutions think for you, then fine, but I have the courage to think for myself and to rely on a little something called "the gut".

I actually tried to post something almost identical to this from my phone while I was at work. Then my phone froze, and then it died, and I reminded myself that ten years from now phones will no longer do that unless you're playing online video games on them.

Sun Jan 09, 2011 9:31 pm

firefly

Joined: 27 Sep 2002
Posts: 3990
Location: Montreal

Breakreep, Thank you for taking the time to copy/paste all this information. I'm happy that this is turning into an actual discussion instead of "you're stupid", "no, you're stupid" nonsense which usually happens when talking about 9/11. Let's hope everyone keeps it that way and goes into the information with open minds.

I read all the parts in bold and noticed it contains information that is contradictory to what hundreds of architects and engineers are saying. It states that temperatures can melt metal if high enough, but didn't provide any evidence that the fires WERE that hot. In fact, hundreds of architects and engineers are providing evidence proving the opposite.

1. Slow onset with large visible deformations
2. Asymmetrical collapse which follows the path of least resistance (laws of conservation of momentum would cause a falling, intact, from the point of plane impact, to the side most damaged by the fires)
3. Evidence of fire temperatures capable of softening steel
4. High-rise buildings with much larger, hotter, and longer lasting fires have never collapsed.

Let's try to keep this discussion focued on information and not bias.

Mon Jan 10, 2011 4:15 pm

firefly

Joined: 27 Sep 2002
Posts: 3990
Location: Montreal

xGasPricesx wrote: Nice try, buddy. But everyone knows that scientific data and facts are just another form of government control. If you want to continue to let the institutions think for you, then fine, but I have the courage to think for myself and to rely on a little something called "the gut".

If this is how you interpreted what I said regarding certain institutions you either have difficulty in understanding basic language or your mind is clouded with bias towards me giving your selective hearing (reading). Or maybe you're just trying to be funny and want to poke fun at the conspiracy theorist. Either way it is not very condusive to having an informative discussion.

I'm a HUGE fan of science - everybody is whether they know it or not. I'm not fond of institutions that monopolize, distort and conceal information. And if you think that no such institutions like this exist then I you gotta open up your eyes and LOOK.

Mon Jan 10, 2011 4:22 pm

xGasPricesx

Joined: 23 May 2008
Posts: 1591

firefly wrote:

xGasPricesx wrote: Nice try, buddy. But everyone knows that scientific data and facts are just another form of government control. If you want to continue to let the institutions think for you, then fine, but I have the courage to think for myself and to rely on a little something called "the gut".

If this is how you interpreted what I said regarding certain institutions you either have difficulty in understanding basic language or your mind is clouded with bias towards me giving your selective hearing (reading). Or maybe you're just trying to be funny and want to poke fun at the conspiracy theorist. Either way it is not very condusive to having an informative discussion.

I'm a HUGE fan of science - everybody is whether they know it or not. I'm not fond of institutions that monopolize, distort and conceal information. And if you think that no such institutions like this exist then I you gotta open up your eyes and LOOK.

I was just being funny. I was done trying to have an "informative discussion" when you had responded to me earlier rather insultingly saying that line about thinking for yourself. I was actually probably done with the conversation before that, since I'm incredibly sick of this topic, but that was really the last nail in the coffin.